1. Field of Technology
The present invention generally relates to the detection of different wavelengths of light and, more particularly, to a spectral sensor of a type which comprises a light receiving unit having an array of light receiving elements, and a filter having a corresponding number of filtering elements of different transmission wavelengths and mounted on the light receiving unit with the filtering elements aligned respectively with the light receiving elements.
2. Description of the Prior Art
The above described type of spectral sensor is disclosed in, for example, the Japanese Laid-open Patent Publication No. 59-20804 and is reproduced in FIG. 13 of the accompanying drawings for the purpose of the dicussion of the prior art, wherein FIG. 13(a) illustrates, in sectional view, an interference filter used in the prior art spectral sensor and FIG. 13(b) illustrates, in top plan view, a light receiving unit having an array of silicon photodiodes.
Referring first to FIG. 13(b), the light receiving unit comprises a silicon substrate 20 having 2n of silicon photodiodes 21a fixedly mounted or formed on one surface of the silicon substrate 20 in a linear row, the assembly constituting a silicon photodiode array chip 22. The silicon substrate 20 has a plurality of, for example, two, rows of terminals 23 formed thereon which are electrically connected with the individual silicon photodiodes 21a by means of respective aluminum electrodes (not shown).
The interference filter 24, best shown in FIG. 13(a), comprises a silicon dioxide (SiO.sub.2) layer 24a having first and second major surfaces opposite to each other across the thickness thereof, only the first major surface being progressively stepped inwardly of the silicon dioxide layer 24a with the thickness thereof varying in stepwise fashion to provide a number of lands while the second major surface remains flat. The first and second major surfaces of the silicon dioxide layer 24a are exteriorly deposited with respective silver films 24b and 24c by the use of any known metal vapor deposition technique, and the silicon dioxide layer 24a is mounted on a glass substrate 25 with the silver film 24c sandwiched between the glass substrate 25 and the silicon dioxide layer 24a.
The lands on the first major surface of the silicon dioxide layer 24a have identical surface areas, the number of such lands being generally selected to be equal to the number of wavelengths of light to be detected, that is, the number 2n of the silicon photodiodes 21a. Conterminous bodies of the silicon dioxide layer 24a that are aligned with the respective lands on the first major surface of the silicon dioxide layer 24a, which bodies have varying thicknesses, constitute respective filter elements F.sub.1 to F.sub.2n. The greater the thickness of the filtering element, the longer the transmission wavelength, and, in the instance as shown, the shortest and longest transmission wavelength ranges may be 400 nm and 700 nm and exhibited respectively by F.sub.1 and F.sub.2n.
In one type, the interference filter 24 is mounted on the photodiode array chip 22 with the glass substrate 25 held in contact with respective light receiving surfaces of the silicon photodiodes 21a, and in another type, the interference filter 24 is mounted on the photodiode array chip 22 in spaced relationship thereto. It is, however, to be noted that, in FIG. 13, the interference filter is shown as exaggerated as compared with the photodiode array chip 22 in the thicknesswise direction thereof.
A light spot l sensed by the prior art spectral sensor of the construction hereinabove described is so selected as to have a spot size .phi..sub.1 slightly greater than the length L.sub.1 of the photodiode array 21 such that, when it impinges upon the photodiode array 21, the photodiode array 21 can be sufficiently covered by the spot of the light beam, as shown in FIG. 14. By way of example, the length L.sub.1 of the photodiode array 21 formed on the silicon substrate 20 of 8.0 mm in length and 3.8 mm in width is 6.0 mm, and, in such case, the size .phi..sub.1 of the light spot l is chosen to be 6.8 mm.
FIG. 15 illustrates, in exploded view, the prior art spectral sensor wherein the assembly of the photodiode array chip 22 and the interference filter 24 is covered with an ultraviolet light cut-off filter 28 and an infrared light cut-off filter 29. More specifically, the spectral sensor shown in FIG. 15 is completed by accommodating the photodiode array chip 22, the interference filter 24, a slit member 27, the ultraviolet light cut-off filter 28 and the infrared light cut-off filter 29 one above the other within a package 26 in the order specified above. The slit member 27 has a generally rectangular slit 27a defined therein, said rectangular slit 27a being of a size equal to that of the photodiode array 21 and is so positioned and so held in position with the slit 27a aligned with the photodiode array 27a. The ultraviolet light cut-off filter 28 is utilized to cut off a secondary wavelength component in the long wavelength range, whereas the infrared light cut-off filter 29 is utilized to cut off a long wavelength component in the short wavelength range.
The prior art spectral sensor of the construction shown in and described with reference to FIGS. 13 to 15 has the following problems. Specifically, since the photodiodes 21a are arranged in a linear row, the radiating light spot l must have a spot size .phi..sub.1 greater than the length L.sub.1 of the photodiode array 21. This in turn requires the use of the substrate 20 having an increased surface area, accompanied by increase in size of the photodiode array chip 22.
While the photodiodes required to be covered by the ultraviolet light cut-off filter 28 is arranged for receiving the light of the long wavelength range and the photodiodes required to be covered by the infrared light cut-off filter 29 is arranged for receiving the light of the short wavelength range, the filters 28 and 29 used in the prior art spectral sensor are each so sized as to cover the entire photodiodes and, therefore, these filters have respective unnecessary portions which work for nothing. Moreover, in the ultraviolet light cut-off filter 28, unless the cut-off characteristic thereof is sharp, there is a possibility in that reduction in output would occur in the short wavelength range to such an extent as to result in difficulty in a highly precise measurement.
In the prior art spectral sensor of the construction shown in and described with particular reference to FIG. 13, since the light receiving surface areas of the respective filtering elements F1 to Fn of the interference filter 24 are identical in size with each other, each silicon photodiode 21a has such a characteristic, as shown in FIG. 8, that the output thereof gradually decreases as the wavelength of light becomes short from about 500 nm, due to the reduction of the photoelectric conversion efficiency and that the output thereof gradually decreases as the wavelength of light becomes long from about 500 nm due to the reduction of the transmission of the interference filter 24.
Since the output varies with the wavelength as hereinabove described, the prior art spectral sensor requires the use of amplifiers of different gains one for each of the silicon photodiodes 21a sensitive to the different wavelengths and, therefore, not only does the circuit as a whole tend to become complicated, but also the S/N ratio varies with the wavelength to such an extent as to result in the difficulty of a highly precise measurement.